Referring now to FIG. 1, an exemplary DC/DC converter 100 is shown. Voltage source VIN and capacitor Cin are connected across conductors 136 and 140. One end of an inductor L1 is connected to the conductor 136 and an opposite end is connected to one end of a primary winding of a transformer T1. One end of an inductor L2 is also connected to the conductor 136 and to the one end of the inductor L1. An opposite end of the inductor L2 is connected to an opposite end of the primary winding of the transformer T1.
A first end of a switch S1 is connected to the opposite end of the inductor L1 and to the first end of the primary winding of the transformer T1. An opposite end of the switch S1 is connected to the conductor 140. A first end of a switch S2 is connected to an opposite end of the inductor L2 and to an opposite end of the primary winding of the transformer T1. An opposite end of the switch S2 is connected to the conductor 140.
A first end of a secondary winding of the transformer T1 (with inductance LLKG) is connected to a second terminal of a diode DB1 and a first terminal of a diode DB2. An opposite end of the secondary winding of the transformer T1 is connected to a second terminal of a diode DB3 and a first terminal of a diode DB4. A first terminal of the diode DB3 and a first terminal of the diode DB3 are connected to a conductor 148. A second terminal of the diode DB2 and a second terminal of the diode DB4 are both connected to a conductor 152. A capacitor CB1 is connected across the conductors 148 and 152. VOUT is taken across capacitor CB1.
Referring now to FIG. 2, an equivalent circuit 200 for the DC/DC converter circuit of FIG. 1 is shown. A switch SA is connected to a current source 12. A body capacitance CDS is connected across switch SA. Inductance LLKG is also connected in series with voltage VSP across switch SA and the body capacitance CDS. The circuit leakage inductance, LLKG, and the body capacitance CDS of the switch SA form a resonant circuit. The resonant circuit has a peak voltage that is a function of I*Z0+Vsp, where Z0=(Llkg/CDS)0.5 and Vsp is the reflected output voltage of the converter.
Referring back to FIG. 1, initially the switch S1 is conducting and inductive energy is stored in the inductances L1 and L2. The switch S1 is subsequently turned off and a step current with magnitude of I flows into the transformer T1 through the leakage inductance Llkg (and the switch S2). The DC/DC converter 100, as shown in FIG. 1, generates large voltage spikes across the switches S1 and S2 during turn off due to the transformer leakage inductance LLKG.
Voltage snubber circuits may be used to limit the voltage spikes across the switching devices. In FIG. 3, a DC/DC converter 300 with a peak clamp snubber circuit is shown. Voltage source VIN is applied across conductors 304 and 308. A first end of an inductor LC1 is connected to the conductor 304. An opposite end of the inductor LC1 is connected to a first end of a primary winding of a transformer T2. A first end of an inductor LC2 is also connected to the input conductor 304 and to the first end of the inductor LC1. The opposite end of the inductor LC2 is connected to the opposite end of the primary winding of the transformer T2. A first end of a switch SC1 is connected to the opposite end of the inductor LC2 and to the opposite end of the primary winding of the transformer T2.
The opposite end of the switch SC1 is connected to the conductor 308. A first end of a switch SC2 is connected to the opposite end of the inductor LC1 and to the first end of the primary winding of the transformer T2. The opposite end of the switch SC2 is connected to the conductor 308.
The anode of a diode DC1 is connected to the first side of the primary winding of the transformer T2 and to one side of the switch SC2. The anode of the diode DC2 is connected to the opposite side of the primary winding of the transformer T2 and to the first side of the switch SC1. The cathode sides of the diodes DC1 and DC2 are both connected to one end of capacitor CC1 and one end of resistor RC1. Opposite ends of capacitor CC1 and resistor RC1 are both connected to input conductor 304.
The first end of the secondary winding of the transformer T2 is connected to the second terminal of a diode DD1 and the first terminal of a diode DD2. The opposite end of the secondary winding of the transformer T2 is connected to a second terminal of a diode DD3 and a first terminal of a diode DD4. The first terminals of the diodes DD1 and DD3 are both connected to a conductor 312. The second terminals of the diodes DD2 and DD4 are both connected to a conductor 316. A capacitor CD1 is connected in parallel across the conductors 312 and 136. Output voltage is provided across the conductors 312 and 316.
Waveforms of snubber circuit 300 are shown in FIG. 4. Reference number 402 shows current through switch SC1. Reference number 404 shows voltage across inductor LC2. Reference number 406 shows voltage across switch SC1. FIG. 4 shows that the peak voltage across the switch is limited to a preset value that is controlled by CC1. Overshoot voltage is limited by the clamp capacitor voltage of the snubbing circuit 300. Disadvantages of the approach shown in FIGS. 3 and 4 relate to component count, component cost, circuit size and power dissipation.
Referring now to FIG. 5, a DC/DC converter 500 with a snubber circuit is shown. A voltage source VIN is applied across conductors 504 and 508. A first end of an inductor LE1 is connected to the conductor 504. The opposite end of the inductor LE1 is connected to a first end of a primary winding of a transformer T3. A first end of an inductor LE2 is also connected to the conductor 504 and to the first end of the inductor LE1. The opposite end of the inductor LE2 is connected to the opposite end of the primary winding of the transformer T3. A first end of a switch SE1 is connected to the opposite end of the inductor LE1 and to the first end of the primary winding of the transformer T3. The opposite end of the switch SE1 is connected to the conductor 508. A first end of the switch SE2 is connected to the opposite end of the inductor LE2 and to the opposite end of the primary winding of the transformer T3. The opposite end of the switch SE2 is connected to the conductor 508.
A first end of capacitor CE4 is connected to the conductor 504 and the opposite end is connected to the conductor 508. An inductor LE3 is connected to the conductor 508 and to the anode end of diode DE5. The cathode of diode DE5 is connected to one end of a capacitor CE2. The other end of the capacitor CE2 is connected to the opposite end of an inductor LE1, the first end of the primary winding of transformer T3, and the first end of switch SE1.
An inductor LE4 is connected to conductor 508 and to the anode of a diode DE6. The cathode of the diode DE6 is connected to one end of a capacitor CE5. The other end of the capacitor CE5 is connected to the opposite end of an inductor LE2, the opposite end of the primary winding of the transformer T3, and the first end of the switch SE2. The anode of a diode DE2 is connected to the cathode end of diode DE5. The anode of a diode DE3 is connected to the cathode end of diode DE6. The cathode ends of the diodes DE2 and DE3 are joined to the first end of a capacitor CE4.
A first end of a secondary winding of the transformer T3 (with inductance LIKG) is connected to the anode of a diode DF1 and the cathode of a diode DF2. The opposite end of the secondary winding of the transformer T3 is connected to the anode of a diode DF3 and the cathode of a diode DF4. The cathode of the diode DF1 and the cathode of the diode DF3 are both connected to the conductor 512. The anode of the diode DF2 and the anode of the diode DF4 are both connected to a conductor 516. A capacitor CF1 is connected across output conductor 512 and output conductor 516.
In FIG. 6, peak voltage characteristics 600 across one of the switches of the snubber circuit 500 are shown. Reference number 602 shows current through the switch SE1. Reference number 404 shows voltage across the switch SE1. In FIG. 7, voltage overshoot 700 for circuit 500 is shown. Circuit 500 requires a snubber capacitance for carrying large pulse current. Accordingly, disadvantages in circuit 500 also relate to component count, component cost, and circuit size.